Multiple methods exist for measuring the thickness of moving sheet such a paper. One common method is a direct thickness measurement using contacting guides, which skim along the two surfaces of the sheet. Since this method contacts the sheets, the method is limited by the strength of the material being measured. With fragile sheets such as tissue, there is a tendency for contacting gauge to snag the sheet surface causing flaws in the sheet or causing the sheet to tear. Additionally, sheets can damage the caliber sensor due to abrasion. Additionally the accuracy of the contacting is reduced as contaminants are built up on the contacting elements, which can occur with coating sheets containing recycled materials.
Another common method is a non-contacting inferential method in which radiation absorption by the sheet is used to determine the thickness of the sheet. The disadvantage of using the inferential thickness measurement with radiation absorption is that if the density of the sheet is not as predicted there can be errors in the calculated thickness.
A number of systems suggest the use of lasers to measure the thickness of moving web. Such laser systems are described in the U.S. Pat. No. 5,210,593 to Kramer and U.S. Pat. No. 4,276,480 to Watson. In such system the laser source on both sides of the web are used to determine a distance on one side of the sheet. The distances are added together and the result is subtracted from a known distance value between the two lasers to determine the sheet thickness.
An improved method is used to compensate for the possibility of changes in the distance between the two laser sensors. A prior art system is shown in FIG. 1. In this example, a Z Coil sensor 102 is used to detect the distance between the laser sensor 104 and the laser sensor 106. Typically, the Z Coil sensor comprises of an RF coil at the upper sensor enclosure that is fixed with respect with the top sensor 104 and flat topped metal dome target at the lower sensor closure that is fixed with respect to the lower laser sensor 106. The coil is typically symmetrically placed with the upper and lower laser to avoid alignment effects that can otherwise arise from a relative tilt between the upper and lower laser sensors assemblies.
For thickness measurement applications such as paper and plastic sheets, the accuracy requirement for relative position thickness changes across the sheet is often less than a micron. This implies that each of the measurement devices needs to be very accurate and stable, especially in terms of their calibration with respect to one another. In a typically scanning implantation, the requirements are very severe since the measured distances can each change significantly (several hundred microns) across the product width due to the movement of the sheet between the lasers or changes in the distance between the lasers as the scanning sensor enclosures move back and fourth.
Laser sensors and Z sensors have some level of drift over time due to temperature or ageing effects. Even if a device is perfectly calibrated at time zero, it will tend to drift out of perfect alignment in the course of hours, days or weeks.
Currently, calibration measurements are done by halting the normal sensor operations and removing the devices from the sensor for recalibration on a periodic basis using a highly accurate means such as an interferometer. The calibration needs to be done at each mill where a sensor installed and skilled people need to be trained at each mill to carry out this procedure. This can be a very expensive proposition.
It is difficult to know when recalibration is needed. This need is often known only after the sensor has already produced inaccurate thickness measurements. Further, there is no guarantee that a calibration done off-line remains accurate when the devices are remounted on to the caliber. For example, a RF coil Z sensor is a proximity sensor that is extremely sensitive to the presence and orientation of near by metal objects. Even tiny differences in the nature and surroundings of the sensor can lead to unacceptable errors in the Z sensor readings.